Had everything gone to plan, Fiona MacKeown should have been sitting in a courtroom in Panaji, Goa, on Friday afternoon, giving evidence for the prosecution in the case of the two men charged with killing her daughter, Scarlett Keeling.

Instead she is back home on her smallholding near the village of Bradworthy in Devon, cursing the Icelandic volcano that spewed ash into the skies of northern Europe, grounding her flight to India and confounding her hopes of testifying.

The 45-year-old can barely keep the frustration out of her voice. This, after all, was meant to be the culmination of her efforts over the past two years to get justice for her daughter, whose bruised, semi-naked body was found on Anjuna beach in north Goa while on a family holiday in February 2008.

When the police dismissed the death of the 15-year-old as a drowning, MacKeown dug her heels in, enlisting a dogged local lawyer, Vikram Varma, to dismantle the official account.

Amid allegations of cover-up and corruption, they managed to get the case reopened, with a second post-mortem examination concluding that Scarlett had not only met a violent death, but that she had also consumed a cocktail of drugs and had been raped.

But by the time the authorities arrested two local men – Placido Carvalho and Samson D'Souza – on charges including culpable homicide, sexual assault, outraging modesty and destroying evidence, the spotlight was already turning on MacKeown.

Questions were being asked about why she had left Scarlett with a friend of the family while travelling further along the coast with her six siblings. There were critical reports about the family's unconventional lifestyle. And then, to cap it all, MacKeown was arrested and charged in the UK with benefit fraud. At a court hearing last month she admitted falsely claiming £19,000 and was warned to expect a custodial sentence.

Now she can only wait and hope that Varma can persuade the Indian prosecution to reschedule her evidence in the next few days to allow her to testify before she is due in court in the UK to be sentenced on 27 April.

"I don't mind talking about it," she says of the fraud charges when she talks to the Observer by telephone. "I haven't got anything to hide. I'm looking into raising the money to pay it back and I hope to go to court with some information about what I can do to pay it back, and just hope for the best really."

MacKeown says she used the money to fund the hobbies of her three eldest children – including Scarlett's horse riding. "None of them had a relationship with their dad. I'd just come out of an abusive relationship and felt very guilty about what I had put them through. It kind of felt the way of making it up to them," she says.

"It is not like I was living a flash lifestyle with it. Obviously it was not the right thing to do and I regret doing it."

She has not given up hope of getting to Goa this week, but she faces the very real prospect of having to give evidence by video link from prison.

After all that she has been through in the past two years, that would be a bitter blow, even if giving evidence in person would mean returning to the state where the murder took place.

"Of course it is painful, it opens up old wounds all the time, every time I see a picture in the paper," she says. "But I'm determined not to let them bury things. I think they are burying the police corruption. I'm worried that if the police who were involved in it walk free, then so will the criminals. To be honest, I don't know if there will be justice. I'd love to be there hearing every single detail, because there does seem to be stuff that we don't know."

Among the early disclosures from the trial is the discovery that one of the first police officers on the scene noticed marks on the body.

"I know now that they knew they were there to start with, that it was all a big lie," she says.

She has good reason to be surprised, given that the police initially dismissed any suggestion of foul play. It was only when she met Varma and found he was prepared to back her suspicions that the official report began to unravel.

"It was all a blur," she says. "It was Vikram really that drove it. I would never have done it on my own. I would not have known where to start. I didn't know that you have to apply for a child's death to be treated as a murder. I just thought the police would work it out when she was found battered and bruised and naked, but they didn't."

MacKeown believes that the accused were probably responsible for her daughter's death, but suspects that there is a larger, more complicated picture yet to emerge, possibly involving the state's thriving drug trade.

"The police were paid off, without a doubt. They tried to make a murder look like an accident," she says. "I don't think the Indian government really wants the truth to come out. I think it would be embarrassing for them. They are either protecting the police officers or protecting the drug trade or the image of Goa."

It is allegations like those that have made MacKeown a hate figure for some in authority in Goa. They have turned their ire on her, questioning her lifestyle and her decision to leave Scarlett behind with her 25-year-old boyfriend, Julio Lobo, and his aunt. MacKeown admits the criticism hurt, but insists it was undeserved.

"We'd had a really lovely week with her before," she says. "It wasn't a decision that was made quickly. She was going to be staying at his aunt's house. I left her in the care of a 25-year-old man who in my mind was acting responsibly. He was getting himself out of the beach bum lifestyle."

Scarlett wanted to stay behind with Lobo, she said, because he was taking her on trips to tourist attractions.

"It seemed to make sense to me. I liked Julio. I had no reason not to. People were saying I had left my daughter with her 25-year-old Indian boyfriend, but it didn't happen like that. Comments like that were really hurtful, but they were made on incorrect facts. I know the people around me know I'm not a negligent parent, my children are my life. I was completely convinced that it was a safe place to be."

As interest in the case grew, reports began to circulate about Scarlett's apparently increasingly wayward lifestyle. In her diary she refers to taking drugs at a farewell party before leaving Devon, but MacKeown insists that this was not the girl she knew.

"She would sit and have a beer with us, but she wasn't off the rails," she says. "It was only a few weeks that we were down the coast. I can't claim to know everything she was up to, in fact she was dabbling a bit in this and a bit in that, but there wasn't time for her to build up this big drug problem."

MacKeown is not nervous at the prospect of returning to India. "I was blown away by India," she says. "My children talk very fondly about India, of the good times we had there. It is not a big black shadow that we don't talk about. They talk about all sorts, the food, some of the creatures they found. They talk about Scarlett there a lot."

She has been back three times since her daughter's death. "I would visit where she was found if I thought it would be safe," she says. "After she was found we stayed there for the first two weeks before it got really heavy. There was a strange sense of comfort about being there. I didn't feel she was still there or anything, but she loved it in Anjuna."

That seems at odds with Scarlett's diaries, which become darker towards the end. The final entries featured a sketch of a gallows, with her lamenting that her boyfriend does not love her and thinks that she only wants him for sex and money. "I'm stuk [sic]" she wrote: "I want to go home."

But MacKeown maintains that the teenager was struggling to decide what to do. "I don't think she was unhappy, I think she was trying to reach a decision whether to come home with us or whether to travel. We had one argument, but it was nothing to do with her staying behind – it was about not doing her washing."

Two years on, Scarlett's body still lies in the mortuary in Exeter, the coroner unwilling to release it until the completion of the Indian legal process.

"I'm very busy getting on with our lives and trying to move forward," MacKeown says. "But it is very difficult when she is still not buried. It makes it harder to move on, doesn't it?"